Recently, rapid processing has been demanded for a light-sensitive material for a color paper in order to finish a large amount of prints in a short delivery time. As one method, there is known a method to enhance the speed of color developing by the use of a silver chloride emulsion or a silver bromochloride emulsion having a high silver chloride content as a silver halide emulsion. For example, the above-mentioned technology is disclosed in U.S. Pat. Nos. 4,183,756 and 4,225,666 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 26589/1980, 91444/1983, 95339/1983, 94340/1983, 95736/1983, 106538/1983, 107531/1983, 107532/1983, 107533/1983, 108533/1983 and 125612/1983.
However, silver chloride emulsions or silver bromochloride emulsions having a high silver chloride content have problems in that fogging is noticeable, sensitivity is low and fluctuation in sensitivity and gradation due to reciprocity law failure, namely due to exposure intensity, is great. In order to solve the above-mentioned problems, various studies have been conducted. In Japanese Patent O.P.I. Publication Nos. 139323/1976 and 171947/1984, technologies to improve processing stability and reciprocity law failure by adding compounds containing a metal in the 8th group of the periodic table are disclosed.
However, the above-mentioned technologies are not sufficient to solve the above-mentioned problems on silver chloride or silver halide having a high silver chloride content. In addition, in Japanese Patent O.P.I. Publication No. 183647, a technology to enhance sensitivity, to improve reciprocity law failure and also to improve fluctuation of sensitivity and gradation caused by a change of temperature in exposure is disclosed.
It is commonly known that doping of iridium compounds is effective for improvement in reciprocity law failure of silver halide emulsions. In Japanese Patent O.P.I. Publication No. 4935/1968, a technology to obtain small fluctuation in terms of gradation in a wide range of exposure time by adding an iridium compound when preparing silver halide grains is disclosed. In addition, there is disclosed a technology to improve sensitivity law failure by removing iridium from the surface of silver halide grains in U.S. Pat. No. 4,997,751.
While most of the above-mentioned problems were solved by the above-mentioned technologies, there still remains a problem that change in sensitivity is great (latent image stability is poor) depending upon intervals of time between exposure and processing. Especially, it is disclosed by Twickey in Journal of Photographic Science Volume 33, p 201 that a technology to dope an iridium compound known as a compound for improving reciprocity law failure noticeably deteriorates latent image stability at an initial stage after exposure, though the problem of reciprocity law failure is surely solved. Therefore, it is not desirable practically. In Japanese Patent O.P.I. Publication No. 135832/1980, it is disclosed that high sensitivity and improvement in reciprocity law failure can be achieved by doping cadmium, lead, copper and zinc. However, according to the study of the present inventors, it has been found that enhancement in high sensitivity and improvement in reciprocity law failure cannot be achieved concurrently and that initial latent image stability is deteriorated.
In Japanese Patent O.P.I. Publication No. 188437/1991, a technology to improve reciprocity law failure and to reduce fluctuation in developing density caused by the period of time from exposure to processing by adding an iridium compound and an iron compound on the surface layer side by 50% or more of grain volume. However, according to the study of the present inventors, it is found that problems still remain practically in that gradation fluctuates due to the period of time from exposure to processing while reciprocity law failure is improved when employing the above-mentioned technologies and that contrast is lowered due to inactive processing.
In Japanese Patent O.P.I. Publication No. 105940/1991, a technology to improve initial latent image stability by doping iridium in a specific region is disclosed. However, further improvement is necessary thereto. In the above-mentioned technology, there is no description about an iridium compound having a bromide ligand. The compound employed is K.sub.3 IrCl.sub.6. In Japanese Patent O.P.I. Publication No. 9034/1992, K.sub.3 IrCl.sub.6 is employed in a pure silver chloride emulsion not containing bromide ion in a comparative example of the Examples. It is very undesirable, being poor in latent image stability and having low contrast compared with K.sub.3 IrCl.sub.6 employed in a comparative example in the same manner.
According to our study, it was found that emulsions containing bromide ion by the use of K.sub.3 IrBr.sub.6 have especially excellent latent image stability. In addition, though Japanese Patent O.P.I. Publication discloses an iridium compound having a chloride ligand, no description about a bromide ligand is observed at all.
In addition, when iridium compounds having a chloride ligand are employed, it was found that silver chloride, especially having a high silver chloride content ratio, and silver bromochloride are not desirable because they cause tailing phenomenon (a phenomenon wherein density is enhanced slightly on the amount of exposure at an extremely low exposure region) producing blurred photography.
Studies for employing polyvalent metal compounds in a silver halide emulsion have long been tried. In Basis for Photography edited by Japan Photographic Society, published by CORONA PUBLISHING CO., LTD, 1978 on page 545, a technology is given to dope Cd.sup.2+, Pd.sup.2+, Cu.sup.2+ or trivalent metal in order to produce electron trap. In Research Disclosure Journal Volume 176 (December of 1978) RD-17643, a technology is given to prepare a direct print emulsion in the presence of tin, lead, copper, cadmium, bismuth, magnesium, rhodium and iridium. U.S. Pat. No. 3,923,513 describes that 4-valent ions such as lead, antimony, bismuth, arsenic, gold, iridium, rhodium, platinum, osmium and iridium are useful for internal-image emulsions described therein. Japanese Patent O.P.I. Publication No. 285941/1989 describes preventing fluctuation in sensitivity and gradation caused by a period of time by the aquation of noble metals of the 8th group. Japanese Patent O.P.I. Publication No. 56238/1990 describes composition distribution of AgCl and metals of the 8th group. Japanese Patent O.P.I. Publication No. 20852/1990 describes silver halide emulsions containing a complex of transition metals having a nitrosyl or thionitrosyl ligand. Japanese Patent O.P.I. Publication No. 20853/1990 describes enhancement of sensitivity achieved by the use of osmium, iridium, rhenium and ruthenium complexes having a cyano ligand. Japanese Patent O.P.I. Publication No. 20855/1990 describes a technology to change the properties of emulsion by adding face-centered cubic grains and Re compound into a silver halide emulsion, wherein the problem of reciprocity law failure at low intensity is reduced by adding rhenium pentabromide complex into a silver bromide emulsion or a silver bromoiodide emulsion.
However, despite the above-mentioned technologies, a satisfactory emulsion in terms of high-contrast gradation property, reciprocity law failure and initial latent image stability has not been obtained.